1. Field of the Invention
The present invention relates to a process for the production of a glass molded article, an optical element produced by the process, and a method of treating a glass. More specifically, it relates to a process for efficiently producing a transparent glass molded article by decoloring a press-molded product, an optical element produced by the above production process and a method of treating a glass, particularly, an optical glass, for coloring and decoloring the glass.
2. Prior Art of the Invention
In recent years, as a technique for mass-producing glass optical elements such as aspherical lenses, increasing interests are invited to a precision press-molding technique of press-molding a glass material made of an optical glass with a mold having a transfer molding surface having a reversal form of an optical element as an end product thereby to obtain a high-precision optical element without cutting and polishing after the press-molding. In the above precision press-molding, press-molding is carried out in a non-oxidizing atmosphere of nitrogen so that the transfer molding surface is not oxidized at a high temperature.
In the field of the above optical glass, there is a demand for reversibly decolorable and colorable glasses. It is general practice to incorporate an ion of a transition metal such as Fe or Co, a colloid of cadmium sulfide, gold or silver, or a sulfide etc. into a glass. In this practice, a glass can be only colored, and a glass that is once colored cannot be rendered colorless or transparent. As a glass that permits reversible control of the color degree, there is known a so-called photochromic glass obtained by incorporating silver chloride. This glass contains a great number of fine particles that are dispersed or precipitated in the glass.
Meanwhile, a P2O5xe2x80x94WO3-containing glass is available as a glass suitable for the above precision press-molding, and this glass is also highly useful as an optical glass that exhibits high-refractivity high-dispersion properties. When optical elements such as a lens made of a P2O5xe2x80x94WO3-containing glass are produced by precision press-molding, there is a problem that a glass that is transparent before the press-molding is colored after the press-molding so that the transparency of the optical glass decreases. The coloring problem caused on a glass article by precision press-molding even if a glass material for the precision press-molding is colorless and transparent takes place not only in the P2O5xe2x80x94WO3-containing glass but also in a P2O5xe2x80x94Nb2O5-containing glass and a P2O5xe2x80x94TiO2-containing glass.
Since the photochromic glass that is reversibly colorable and decolorable contains a great number of fine particles dispersed and precipitated in the glass, it has a problem that it causes light scattering and is sometimes not suitable for use where particularly high homogeneity is required.
The above problems can be all overcome by controlling the color degree of a glass.
Under the circumstances, it is an object of the present invention to provide a process for efficiently producing a transparent glass molded article, particularly a precision press-molded article having a high transmittance, an optical element obtained by said process, and a treatment method of effectively coloring and decoloring a glass, particularly, an optical glass.
For achieving the above object, the present inventors have made diligent studies and as a result found the following. A P2O5xe2x80x94WO3-containing glass, a P2O5xe2x80x94Nb2O5-containing glass and a P2O5xe2x80x94TiO2-containing glass are colored because W ion, Nb ion and Ti ion in the glasses are reduced due to exposure to a non-oxidizing atmosphere when the glasses are precision press-molded in a high-temperature state. Therefore, the P2O5xe2x80x94WO3-containing glass, the P2O5xe2x80x94Nb2O5-containing glass and the P2O5xe2x80x94TiO2-containing glass are colored by exposing the glasses to a non-oxidizing atmosphere (a more remarkable change takes place in a reducing atmosphere) at a high temperature. On the other hand, W ion, Nb ion and Ti ion are oxidized by exposing the glasses to an oxidizing atmosphere in a high-temperature state, so that the color degree can be decreased and, further, that the above-colored glass can be decolored. And, it is difficult to control the color degree by merely heating the glass, and the color degree is greatly influenced by oxidizing and reducing atmospheres to which the glass under heat is exposed.
The present invention has been completed on the basis of the above findings.
That is, (1) according to the present invention, there is provided a process for producing a glass molded article by press-molding a glass under heat in a non-oxidizing atmosphere, the process comprising press-molding a glass containing at least one oxide selected from WO3, Nb2O5 or TiO2, to prepare a glass molded article, and then heat-treating the glass molded article in an oxidizing atmosphere.
(2) According to the present invention, there is also provided an optical element obtained by the above method.
(3) According to the present invention, there is further provided a method of treating a glass, comprising heat-treating a colored glass containing at least one oxide of WO3 and Nb2O5 in an oxidizing dry atmosphere, to decolor the glass.
(4) According to the present invention, there is still further provided a method of treating a glass, comprising heat-treating a glass containing at least one oxide selected from WO3, Nb2O5 or TiO2 in a non-oxidizing atmosphere, to color the glass.